The present invention is generally directed to toner compositions and processes for the preparation of toner resins and toner compositions. More specifically, the present invention relates to melt mixing processes, batch or continuous, and preferably continuous processes such as, for example, reactive extrusion for preparing crosslinked toner resins. The present invention relates to processes for crosslinking reactive linear resins which can be subsequently selected for the preparation of crosslinked toner resins that can be selected for application in heat fixable toners with superior fusing properties and excellent vinyl offset characteristics. In embodiments, the present invention relates to processes for crosslinking reactive linear resins wherein the chemical initiators selected for crosslinking are contained in a nonreactive or reactive polymer resin or a toner additive resin. Advantages associated with the present invention include the avoidance of hazards associated with the handling of dry powdered and liquid chemical initiators.
Toner utilized for development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known, including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black can be utilized, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are known.
To fix the toner to a support medium, such as a sheet of paper or transparency, hot roll fixing is commonly used. In this method, the support medium with a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image. This fixing system can be very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images resulting, for example, in blurred images. This undesirable phenomenon is known as offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the Fusing Latitude.
The known hot roll fixing system and a number of toners used therein can exhibit several problems. For example, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Also, toner and image offsetting can be a problem. Moreover, toners containing vinyl type binder resins, such as styrene-acrylic resins, may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image contacts for a period of time a polyvinyl chloride (PVC) surface containing a plasticizer, reference for example vinyl binder covers, and the fixed image adheres to the PVC surface.
There is a need for a toner resin with a fix temperature below 200.degree. C. and preferably below about 160.degree. C., that is a low fix temperature toner resin or a low melting toner resin, toner and toner resins with excellent offset properties and superior vinyl offset characteristics, and processes for the preparation thereof.
To prepare lower fix temperature resins for toner, the molecular weight of the resin may be lowered. Low molecular weight polymers such as amorphous polyester resins and epoxy resins have been used for low temperature fixing toners. For example, toners utilizing polyester resins as binders are illustrated in U.S. Pat. No. 3,590,000 and U.S. Pat. No. 3,681,106, the disclosures of which are hereby totally incorporated by reference herein. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may result in a lowering of the hot offset temperature and, as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of toner blocking during storage.
To prevent fuser roll offsetting and to improve fusing latitude performance of toners, various modifications have been made to toner compositions. For example, waxes, such as low molecular weight polyethylene, polypropylene, and the like, can be added to toners to increase their release properties, reference U.S. Pat. No. 4,513,074, the disclosure of which is hereby totally incorporated by reference herein. However, to sufficiently prevent offset, considerable amounts of such materials may be required in some instances, resulting in detrimental effects such as the tendency for toner agglomeration, undesirable free flow properties and destabilization of toner charging properties.
Modification of the binder resin structure, for example by branching, crosslinking, and the like, when using conventional polymerization reactions may also improve offset resistance. For example, in U.S. Pat. No. 3,681,106 a process is disclosed whereby a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is by crosslinking during polymerization. In U.S. Pat. No. 3,941,898, the entire disclosure of which is hereby totally incorporated by reference, there is illustrated, for example, a crosslinked vinyl type binder polymer prepared with conventional crosslinking. Similar disclosures for vinyl type resins are illustrated in U.S. Pat. Nos. Re. 31,072 (a reissue of U.S. Pat. No. 3,938,992); 4,556,624; 4,604,338, and 4,824,750 the disclosures of which are hereby totally incorporated herein by reference. Also, crosslinked polyester binder resins prepared by conventional polycondensation processes for improving offset resistance are illustrated in U.S. Pat. No. 3,681,106.
While improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue with certain crosslinked resins prepared by conventional polymerization, both vinyl type processes including solution, bulk, suspension and emulsion polymerizations; and polycondensation processes. In these processes which operate typically between room temperature and 200.degree. C., monomer and crosslinking agent are added to the reactor. The crosslinking reaction is not very rapid and chains can grow in more than two directions at the crosslinking point by the addition of monomers. Also, there are monomeric units between the crosslinked chains. Three types of polymer configurations are believed to result: a linear and soluble portion referred to as the linear portion, a crosslinked portion which is low in crosslinking density and, therefore, is soluble in some solvents, such as tetrahydrofuran, toluene and the like, and is referred to as the sol; and a portion comprising highly crosslinked gel particles which is not soluble in substantially any solvent, such as tetrahydrofuran, toluene and the like, and is referred to as the gel. The second portion with low crosslinking density (sol) functions primarily to widen the molecular weight distribution of the soluble part which results in an elevation of the minimum fixing temperature of the toner. The presence of monomeric units between the crosslinked chains enables gel swelling in the presence of solvents. Another disadvantage of these processes, which are effected under low shear, that is less than 0.1 kW-hr/kg), is that as more crosslinking agent is used the gel particles or very highly crosslinked insoluble polymer with high molecular weight increase in size. These large gels can be more difficult to disperse pigment in causing unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, with the formed resulting polymers, the toners thereof often evidence vinyl offset.
U.S. Pat. No. 4,533,614, the entire disclosure of which is hereby totally incorporated by reference herein, discloses a loosened crosslinked polyester binder resin which exhibits low temperature fix and some offset resistance. Metal compounds were used as crosslinking agents. Similar disclosures are presented in U.S. Pat. No. 3,681,106 and Japanese Laid-open Patent Applications 94362/1981, 116041/1981 and 166651/1980, the disclosures of which are hereby totally incorporated by reference herein. As indicated in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, with color toners other than black, such as cyan, metal complexes can adversely affect the color of the pigments. It is also known that metal containing toner can have disposal problems in some areas, such as for example in the State of California, and metal complexes are often also costly.
U.S. Pat. No. 4,894,308 and U.S. Pat. No. 4,973,439, the disclosures of which are totally incorporated herein by reference, disclose, for example, extrusion processes for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into a binder resin in the extruder. However, in each of these patents there is no suggestion, it is believed, of a chemical reaction occurring.
An injection molding process for producing crosslinked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736, the disclosure of which is totally incorporated herein by reference, in which polyolefin or polyvinyl chloride resin and crosslinking agent were mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the crosslinking reaction occurred at increased temperature and pressure, and at low or zero shear.
In U.S. Pat. No. 4,089,917, the disclosure of which is totally incorporated herein by reference, an injection molding and crosslinking process is disclosed in which polyethylene resin and crosslinking agent were mixed in an extruder and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. The crosslinking reaction in the reaction chambers was accomplished at low or zero shear, and the final product is a thermoset molded part, and thus, is not considered useful as a toner resin.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293, the disclosure of which is hereby totally incorporated herein by reference, in which polyurethane precursor systems were crosslinked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress was greater than a critical value to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, and thus, is not considered useful as a toner resin.
The processes disclosed in U.S. Pat. Nos. 3,876,736; 4,089,917 and 4,990,293 are not believed to be reactive extrusion processes since, for example, crosslinking occurs in a die or a mold, and not in an extruder. These processes are for producing engineering plastics such as thermoset materials which cannot be remelted once molded, and thus are not believed to be effectively suitable for toner applications.
In copending U.S. patent application Ser. No. 07/814,641 (D/91117) and U.S. Pat. No. 5,227,460 (D/91117Q), the disclosures of which are totally incorporated herein by reference, a process, toner resin composition, and toner composition are disclosed in which partially crosslinked thermoplastic binder resins for toners with low fix temperature, hot offset temperature related to the degree of crosslinking, and which exhibit minimal or substantially no vinyl offset are produced by reactive extrusion. In this process, a reactive resin, such as, for example, an unsaturated linear polyester resin, is crosslinked in the molten state under high temperature and high shear conditions using a chemical initiator such as, for example, certain organic peroxides, as a crosslinking agent in a batch or continuous melt mixing device such as, for example, an extruder. Processing of the aforementioned dry powdered chemical initiators in their virgin state can present hazards, such as, for example, nuisance dust and explosions in dry blending operations and in feeding to a melt mixing process. Injection of a liquid initiator to a melt mixing process also presents hazards, such as for example explosions. Further, if the virgin chemical initiator, dry powder or liquid is heated too quickly in the reactive extrusion process before being dispersed in the reactive resin, parallel reactions with crosslinking can occur resulting in the formation of undesirable byproducts. In addition, premature crosslinking can occur resulting in poor dispersion of the gel particles in the toner resin. These and other disadvantages are avoided or minimized with the present invention, especially when in embodiments there is selected a pelletized peroxide, available from Polyvel of New Jersey, and which peroxide can be effectively formulated into powders of various mesh sizes.